In recent years, various types of information devices such as television sets, mobile phones, PDAs (Personal Digital Assistants) have gained prevalence and entered into people's lives. Thus, users need to manipulate information devices in many scenes of their usual lives. Usually, in realizing a device manipulation, a user utilizes a hand to input an input command via an input means (interface section) such as a button. However, in situations where both hands are full because of tasks other than a device manipulation, e.g. household chores, rearing of children, or driving, it is difficult to make an input by using an interface section and it is impossible to realize a device manipulation. Therefore, there are increasing needs of users to manipulate information devices in every kind of situation.
In answer to such needs, input means utilizing biological signals from a user has been developed. For example, Non-Patent Document 1 discloses an electroencephalogram interface that utilizes an event-related potential of electroencephalogram for distinguishing an option which a user wishes to select. To specifically describe the technique described in Non-Patent Document 1, options are randomly highlighted, and a P3 component of an event-related potential which appears about 300 ms after a point in time that an option was highlighted is utilized to enable distinction of the option which the user wishes to select. According to this technique, a user is able to identify an option which he or she wishes to select, without using a hand.
As used herein, an “event-related potential” refers to a transient potential fluctuation in the brain which occurs in temporal relationship with an external or internal event. An electroencephalogram interface utilizes an event-related potential which is obtained from a stimulation to the visual sense as an external event. For example, within the event-related potential for a visual stimulation, a so-called P3 component may be utilized to perform processing such as switching of channels, selection of a program genre of which viewing is desired, and sound volume level adjustment. The “P3 component” refers to a positive component of the event-related potential which appears in a time slot of 250 ms to 500 ms after a target stimulation is presented, regardless of the type of sensory stimulation such as auditory sense, visual sense, or somatic sensation.
For an application of the event-related potential to an interface, it is important to distinguish the event-related potential (e.g., the P3 component) of a subject with a high accuracy. Therefore, it is necessary to accurately measure a biological signal and accurately distinguish the measured biological signal with an appropriate distinction technique.
There are two factors that may lower the distinction ratio. A first factor is that an electroencephalogram component (e.g., the P3 component) which is used for an electroencephalogram interface has a low signal-to-noise ratio (S/N ratio) and a low distinction technique accuracy, so that a highly accurate distinction has not been realized. Regarding this factor, a method for removing the noise mixed in the electroencephalogram and a highly accurate distinction method are both being under development.
For example, Patent Document 1 discloses a technique of improving the distinction ratio which uses a band-pass filter to remove, among the noises contained in the electroencephalogram, noises that are mixed at a frequency different from the frequency of a subject of distinction (event-related potential), e.g., noises on commercial power, and thereafter performs distinction. Patent Document 2 discloses, as a technique of removing noises from living organisms which are difficult to remove with a simple frequency filter, e.g., electro-oculographic potential, a technique of excluding any samples containing an electro-oculographic potential from the subject of distinction, thus obtaining an improved distinction ratio.
A second factor that may lower the distinction ratio is that, depending on the state of the test subject, for example, no electroencephalogram component that serves as a subject of distinction may appear in the electroencephalogram of the test subject, thus making it impossible to perform distinction. Regarding this factor, conventional experiments under laboratory room conditions have adopted a technique of controlling the state of the test subject by instructing the test subject to concentrate on the task by providing a laboratory room which is free of disturbances, or causing the test subject to press a confirmation button, etc., thus allowing a response to steadily appear.                [Patent Document 1] Pamphlet of International Laid-Open No. 2005/001677        [Patent Document 2] Japanese Laid-Open Patent Publication No. 10-146323        [Non-Patent Document 1] Emanuel Donchin and two others, “The Mental Prosthesis: Assesing the Speed of a P300-Based Brain-Computer Interface”, TRANSACTIONS ON REHABILITATION ENGINEERING 2000, Vol. 8, June 2000        